A conventionally known apparatus of an electrodeless discharge lamp of this kind comprises: an airtight container bulb made of a transparent material and enclosing a discharge gas such as mercury or argon; and an induction coil apparatus which is contained in a hollow portion (hereafter referred to as cavity) provided in this bulb, and which generates a high frequency electromagnetic field by conducting a high frequency current to excite the discharge gas so as to emit light, as shown, for example, in Japanese-translated Laid-open Publication of International Patent Application Hei 11-501152. This induction coil apparatus is formed of an assembly body (hereafter referred to as coupler) of: a coil for generating electromagnetic energy by current conduction; a core made of a soft magnetic material; and a thermal conductor (hereafter referred to as cylinder) for heat release. This kind of electrodeless discharge lamp has advantages that it has a long life because it has no electrode, and that it has good lighting-up responsiveness, and further that it is easy to airtightly seal a glass bulb, and is easy to assemble. However, at the same time, the core positioned in the cavity and formed of a coil and a soft magnetic material is exposed to heat from the bulb while lit. Accordingly, loss due to an increase in coil resistance and reduction in reliability of a coil insulation material become problems, which requires design for heat exhaustion to be devised.
In the electrodeless discharge lamp shown in the above-described Japanese-translated Laid-open Publication of International Patent Application Hei 11-501152, attention is paid to the relationship of the arrangement between a ferrite core and a cylinder in order to increase the heat exhaustion effect by thermal conductor. More specifically, it is described that the heat exhaustion effect is increased by arranging an aluminum-made cylinder in a manner to contain a ferrite core, and by controlling the cross-sectional area ratio between the core and the cylinder.
However, in this electrodeless discharge lamp, a resin-made bobbin for winding the coil is provided to cover the core and the cylinder, in which the resin-made bobbin is poor in thermal conductivity, and in addition cannot prevent an air layer from intervening therebetween when mounted on the core and the cylinder. Air is very poor in thermal conductivity. As a result, it is not possible to effectively exhaust heat of the coil received from the heat-generating bulb. Thus, the coil temperature markedly increases, making it impossible to prevent the reliability of the coil insulation from being reduced. Further, a divided ferrite core is used, which causes the shape of the cylinder to be complex in order to fix such core. Furthermore, although it is possible to consider a structure in which the coil is wound around the core without using a resin-made bobbin, the positional accuracy of the coil is likely to decrease, making it likely that the lighting performance is caused to vary.